The present invention relates to a method for locating a fault in a high voltage transmission line insulator by measuring the electric field perturbation caused by that fault. Specifically, in the present method, a small 60 Hz sperhical dipole or other nonintrusive, noncontacting electric field sensing device may be safely employed under a wide range of environmental conditions to detect insulator failure.
High voltage transmission line insulators initially have a relatively high dielectric constant. Over time, however, various environmental factors in combination with the electric fields generated by the transmission lines can cause changes in this constant and a corresponding reduction in the quality of insulation provided. For example, extremes of heat and cold can lead to fissures in the insulating material. In arrid regions, high winds carrying sand or other airborne particles can abrade the insulator's protective ceramic covering. Furthermore, very humid conditions or salty air in coastal areas may contribute to insulator failure through the deposition of salt on insulator surfaces. Any of the above-mentioned environmental factors can lead to areas of conductivity which substantially change the insulator's dielectric constant and, therefore, could eventually cause a failure leading to a power outage.
The ability to monitor insulator quality in the field has been hindered by a lack of suitable instrumentation and test methods. The need for ongoing insulator monitoring, however, is steadily growing as right-of-way problems and costs lead to ever increasing numbers of dedicated transmission lines with no backup in case of outage.
Although faulty insulators and faulty insulator strings may be detected while they are in service, many of the current test procedures require contact with the insulator and, therefore, tedious precautionary measures must be employed. Noncontact testing methods also may be employed; however, such methods generally are not as accurate as contact methods. In general, the methods currently employed for faulty insulator detection are based upon the measurement of the voltage gradient across the individual units of a string of suspension insulators or across the parts of multi-part pin type insulators. For safety reasons, none of the existing contact test methods should be used in wet weather.
One of the most common non-contact methods of detecting faulty insulators is visual inspection to determine whether the insulator is chipped, cracked, or portions of the skirt are broken off. Also, visual inspections may be used to detect arcing or sparking from the conductor along the length of the insulator string.
Faulty insulators also may be detected by special radio inteference locators consisting essentially of a sensitive battery-operated receiver coupled to either a directional loop or which antenna. The latter type may be attached to a "hot line stick" to enable close investigation of the insulator under test.
Mechanical devices, such as "buzz sticks", may be used to detect insulator faulting by bridging an insulator and allowing the test personnel to observe any arcing or sparking, thereby indicating a faulty insulator. Use of such a device, however, requires experienced personnel to conduct the test and to obtain reliable data. Further, methods utilizing these devices are limited to relatively low voltage lines, e.g., no higher than 230 kV. Moreover, the weather must be fair and the insulator must be dry before any attempt is made at conducting the test.
A number of fault detection methods based on live-line insulator test devices are currently available. These devices require bridging an insulator, measuring the voltage distribution on it and comparing the values to the others in the insulator string. These units are equipped with a meter or buzzer mechanism which informs the linemen of questionable units. Again, these systems are limited by environmental conditions and can only be used on lower voltage lines, e.g., no higher than 230 kV.
Another prior art insulator fault detection system pulses an insulator with an 80 kV pulse. If the insulator is defective, the operator hears an audible buzz from the instrument's associated circuitry. This system has some inherent problems, however, as evidenced by a number of documented cases in which this device caused an insulator to explode when the high voltage was applied, killing or seriously injuring even experienced linemen.
As was discussed above, most of the prior art methods and devices for detecting faults in insulator strings require physical contact of the testing device with the insulator. Physical contact of the device to be tested, however, often contributes to inaccuracies in test results and, in many cases, to an increased hazard for the test system operator. Prior art noncontact test methods, such as radio interference and visual inspection, generally provide a greater margin of safety for the operator but a lower degree of accuracy. The nonintrusive, noncontacting test method described hereinbelow represents an improvement over the test methods described above both in terms of increased accuracy and increased safety.